\documentclass[tikz,border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage{tikz-feynman}
\begin{document}

	\begin{tikzpicture}
		\begin{feynman}
			%% fig a
			\vertex (a1) at (0,0);
			\vertex[right =1cm  of a1] (a2);
			\vertex[right =2cm  of a1] (a3);
			\vertex[right =3cm  of a1] (a4);
			\vertex[right =4cm  of a1] (a5);
			\vertex[above =1cm of a2] (a6);
			\vertex[above =1cm of a4] (a7);
			%\node[above =1.5cm  of a5] {$a$};
			\diagram*{
			{ [edge = charged scalar ]
			(a1) --[momentum'={\scriptsize \(k\)}]  (a2),
			(a2) -- [momentum'={\scriptsize \(k+q\)}] (a3),
			},
			% 介子连线
			{ [ edge= photon]
			(a2) --[reversed momentum = {\scriptsize \(q\)}](a6),
			}
			};
		\end{feynman}
	\end{tikzpicture}

\end{document}
